Boron alkyl compounds are known to be capable of initiating free radical polymerizations at room temperature in the presence of oxygen or compounds that supply oxygen. Oxygen needed for initiating polymerization is always present, and need not be added separately. Simple trialkyl boron compounds such as triethyl boron or tri-n-butyl boron have been proposed for this purpose. The use of trialkyl boron compounds as polymerization initiators has been disclsed, for instance, in U.S. Pat. Nos. 3,476,727; 3,633,490; and 2,985,633 as well as British Pat. No. 1,113,722. U.S. Pat. No. 4,167,616 discloses reaction products of butadiene and diborane, and their application as polymerization initiators.
Such boron alkyl compounds are also suitable as reaction initiators or as hardeners for reactive adhesives which contain systems based on polymerizable monomers. Methacrylate adhesives in particular are examples of such systems. In addition to acrylic acid esters or methacrylic acid esters, these adhesives contain trialkyl boron compounds, such as triethyl boron or tri-n-butyl boron, as their essential component (see Japanese Application No. 42/14,318 (1967). Such trialkyl boron compounds, however, have the disadvantage that they are readily flammable, which makes handling of such adhesives quite difficult.
Attempts have been made to overcome this disadvantage by reacting the trialkyl boron compounds with 0.3 to 0.9 mole of oxygen; see German Application No. 23 21 215. Attempts have also been made to react the trialkyl boron compounds with amines in order to reduce their spontaneous ignition properties; see Japanese Application No. 45/29,195 (1970).
By the use of these techniques the ignition temperature is shifted into the range of 0.degree. to 70.degree. C.; however, considerable uncertainty remains with respect to the handling of such mixtures. In addition, the reactivity of these boron alkyl derivatives is greatly reduced.
If unlimited quantities of oxygen come into contact with free boron alkyls, oxidation to boric acid esters occurs, with attendant loss of their ability to be used as polymerization initiators. Until now, in order to preserve the activity of boron alkyl compounds as initiators, it was necessary to eliminate all contact with oxygen when using or measuring them, and of course also during their manufacture. Consequently, any required quantity of the boron alkyl material must be packaged by means of an inert gas into totally air tight vessels in order to eliminate the possibility of oxygen entering the storage vessel. The portioned boron alkyls must then be used quantitatively. Due to these complications, the previously described systems are not suitable for a number of applications; for example, they are unsuited for many practical uses of adhesives.
Boron alkyl initiators show substantial advantages over conventional initiators of free radical polymerizations such as the peroxides, hydroperoxides, or azo-compounds. For example, polymerizations initiated by boron alkyls can be carried out at low temperatures. Also, the starter system/hardener system is available in a single component form. Furthermore, the rate of polymerization can be influenced by varying the amount of oxygen available.
The described disadvantages of simple trialkyl boron compounds, and in particular, their spontaneous ignition liabilities may be partially eliminated by the careful selection of more stable boron alkyl compounds. An example thereof is 9-borabicyclo[3.3.1]nonane (9-BBN). However, this dialkyl boron hydride, as well as other more stable boron alkyls, dissolves with great difficulty, or very slowly, in many monomers, especially monomer systems containing ester groups. This places a serious limitation on the use of these known boron alkyl compounds that are more stable when exposed to atmospheric oxygen.